Gas separation using membranes comprising polybenzoxazoles prepared by thermal rearrangement

ABSTRACT

A method of separating components of a gas mixture, the method comprising: passing the gas mixture through a benzoxazole-based polymer membrane at a temperature of from about 30° C. to about 400° C., wherein the benzoxazole-based polymer membrane is represented by the formula: 
     
       
         
         
             
             
         
       
     
     as is defined herein.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No. 12/921,980, filed on Mar. 13, 2008, the disclosure of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a method for preparing a benzoxazole-based polymer by thermal rearrangement which is performed by a simple process and induces thermal rearrangement at relatively lower thermal conversion temperatures to prepare a benzoxazole-based polymer suited for application to gas separation membranes, in particular, to gas separation membranes for small gases, the benzoxazole-based polymer prepared by the method and a gas separation membrane comprising the benzoxazole-based polymer.

BACKGROUND ART

Free-volume elements in soft organic materials have been focused upon to improve membrane separation performance in chemical products as well as for energy conversion and storage applications [P. M. Budd, N. B. McKeown, D. Fritsch, Polymers with cavities tuned for fast selective transport of small molecules and ions, J. Mater. Chem. 2005, 15, 1977; W. J. Koros, Fleming G. K., Membrane-based gas separation, J. Membr. Sci. 1993, 83, 1; S. A. Stern, Polymers for gas separations: The next decade, J. Membr. Sci. 1994, 94, 1].

The free volume element size and distribution play a key role in determining permeability and separation characteristics of polymers. Among typical polymeric membranes, glassy polymers have exhibited good gas separation performance with high selectivity, however, permeability of glassy polymers is poorly suited to practical applications [M. Langsam, “Polyimide for gas separation, in Polyimides: fundamentals and applications”, Marcel Dekker, New York, 1996; B. D. Freeman, Basis of permeability/selectivity tradeoff relations in polymeric gas separation membranes, Macromolecules 1999, 32, 375].

Even though some glassy polymers with ultra-high free volume such as poly(1-trimethylsilyl-1-propyne) (PTMSP), poly(4-methyl-2-pentyne) (PMP), and copolymers of 2,2-bis-trifluoromethyl-4,5-difluoro-1,3-dioxide and tetrafluoroethylene (Teflon® AF amorphous fluoropolymers) exhibited extremely high gas permeability, they still had very low performance in selectivities. [K. Nagai, T. Masuda, T. Nakagawa, B. D. Freeman, I. Pinnau, Poly[1-(trimethylsilyl)-1-propyne] and related polymers: Synthesis, properties and functions, Prog. Polym. Sci. 2001, 26, 721; A. Morisato, I. Pinnau, Synthesis and gas permeation properties of poly(4-methyl-2-pentyne), J. Membr. Sci. 1996, 121, 243; A. M. Polyakov, L. E. Starannikova, Y. P. Yampolskii, Amorphous Teflons AF as organophilic pervaporation materials: Transport of individual components, J. Membr. Sci. 2003, 216, 241].

A great deal of research has endeavored to produce ideal structures having precise cavities for high gas permeability and high gas selectivity. As a result of this research, there has been remarkable development of polymer membranes exhibiting high gas-separation performance. For example, designs for nanocomposites, hybrid materials and complex polymers were considered to impart large free volume to polymers.

Of these, methods to realize intermediate and small cavity size distributions were reported recently [H. B. Park, C. H. Jung, Y. M. Lee, A. J. Hill, S. J. Pas, S. T. Mudie, E. Van Wagner, B. D. Freeman, D. J. Cookson, Polymers with cavities tuned for fast selective transport of small molecules and ions, Science 2007, 318, 254. 38].

Lee et al. suggested that completely aromatic, insoluble, infusible polybenzoxazole (TR-α-PBO) membranes can be prepared by thermally modifying ortho-hydroxyl group-containing polyimide aromatic polymers through thermal rearrangement to molecular rearrangement at 350 to 450° C. [H. B. Park, C. H. Jung, Y. M. Lee, A. J. Hill, S. J. Pas, S. T. Mudie, E. Van Wagner, B. D. Freeman, D. J. Cookson, Polymers with cavities tuned for fast selective transport of small molecules and ions, Science 2007, 318, 254. 38].

TR-α-PBO membranes have advantages of excellent gas separation performance and superior chemical stability and mechanical properties, surpassing the limitations of typical polymeric membranes (i.e., the Robeson's upper bound). [L. M. Robeson, Correlation of separation factor versus permeability for polymeric membranes, J. Membr. Sci., 1991, 62, 165, L. M. Robeson, The upper bound revisited, J. Membr. Sci., 2008, 320, 390]. However, in spite of extremely high permeability in CO₂ separation, TR-α-PBO still exhibits low selectivity for small gases such as hydrogen and helium.

OBJECTS OF THE INVENTION

Therefore, it is one object of the present invention to provide a method for preparing a benzoxazole-based polymer, wherein the method is performed by a simple process and induces thermal rearrangement at relatively lower temperatures.

It is another object of the present invention to provide a poly(hydroxyamide) intermediate suitable for the preparation of the benzoxazole-based polymer.

It is another object of the present invention to provide polybenzoxazole (TR-β-PBO) having morphological and physical properties different from conventional polybenzoxazole (TR-α-PBO).

It is another object of the present invention to provide a poly(hydroxyamide) (PHA) intermediate suitable for the preparation of the polybenzoxazole (TR-β-PBO).

It is another object of the present invention to provide a gas separation membrane comprising the polybenzoxazole (TR-β-PBO) with high permeability and superior selectivity for small gases.

It is another object of the present invention to provide a method of separating gases at high temperatures by passing a gaseous mixture through a gas separation membrane comprising a polybenzoxazole (TR-β-PBO).

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention for achieving the above object, there is provided a method for preparing a benzoxazole-based polymer represented by Formula 1, by thermally treating poly(hydroxyamide) represented by Formula 2, as depicted in Reaction Scheme 1 below:

wherein Ar is a bivalent C₅-C₂₄ arylene group or a bivalent C₅-C₂₄ heterocyclic ring, which is substituted or unsubstituted with at least one substituent selected from the group consisting of C₁-C₁₀ alkyl, C₁-C₁₀ alkoxy, C₁-C₁₀ haloalkyl and C₁-C₁₀ haloalkoxy, or two of more of which are fused together to form a condensation ring, or covalently bonded to each other via a functional group selected from the group consisting of O, S, C(═O), CH(OH), S(═O)₂, Si(CH₃)₂, (CH₂)_(p) (in which 1≦p≦10), (CF₂)_(q) (in which 1≦q≦10), C(CH₃)₂, C(CF₃)₂ and C(═O)NH;

Q is a single bond, O, S, C(═O), CH(OH), S(═O)₂, Si(CH₃)₂, (CH₂)_(p) (in which 1≦p≦10), (CF₂)_(q) (in which 1≦q≦10), C(CH₃)₂, C(CF₃)₂, C(═O)NH, C(CH₃)(CF₃), C₁-C₆ alkyl-substituted phenyl or C₁-C₆ haloalkyl-substituted phenyl in which Q is linked to opposite both phenyl rings in the position of m-m, m-p, p-m or p-p; and

n is an integer of 20 to 400.

In accordance with another aspect of the present invention, there is provided a poly(hydroxyamide) intermediate represented by Formula 2 used in the preparation of the benzoxazole-based polymer Formula 1.

wherein Ar, Q and n are defined as above.

In accordance with another aspect of the present invention, there is provided polybenzoxazole (TR-β-PBO) represented by Formula 3, having a glass transition temperature (Tg) of 377° C. and a d-spacing of 6.0 to 6.10 Å.

In accordance with another aspect of the present invention, there is provided a method for preparing polybenzoxazole (TR-β-PBO, 3) by thermally treating poly(hydroxyamide) (PHA, 8), as depicted in Reaction Scheme 3 below:

In accordance with another aspect of the present invention, there is provided a poly(hydroxyamide) intermediate represented by the following Formula 8 used for the preparation of the polybenzoxazole (TR-β-PBO).

In accordance with another aspect of the present invention, there is provided a gas separation membrane comprising polybenzoxazole (TR-β-PBO) represented by Formula 3 and having a glass transition temperature (Tg) of 377° C.

According to the method of the present invention, polybenzoxazole is simply prepared by thermally converting poly(hydroxyamide) as an intermediate via thermal treatment at low temperatures. The polybenzoxazole thus prepared exhibits superior mechanical and morphological properties and has well-connected microcavities, thus showing excellent permeability and selectivity for various types of gases.

The polybenzoxazole is suited for application to gas separation membranes, in particular, gas separation membranes for small gases, e.g. H₂/CH₄, H₂/CO₂, H₂/N₂, He/N₂, O₂/N₂, CO₂/N₂, and CO₂/CH₄.

In yet another aspect, the present invention provides a method of separating H₂ and CO₂ from a gas mixture, the method comprising: passing the gas mixture comprising H₂ and CO₂ through a benzoxazole-based polymer membrane at a temperature of from about 30° C. to about 400° C., wherein the benzoxazole-based polymer membrane is represented by the formula:

wherein Ar is a bivalent C₅-C₂₄ arylene group or a bivalent C₅-C₂₄ heterocyclic ring, which is substituted or unsubstituted with at least one substituent selected from the group consisting of C₁-C₁₀ alkyl, C₁-C₁₀ alkoxy, C₁-C₁₀ haloalkyl and C₁-C₁₀ haloalkoxy, or two or more of which are fused together to form a condensation ring, or covalently bonded to each other via a functional group selected from the group consisting of O, S, C(═O), CH(OH), S(═O)₂, Si(CH₃)₂, (CH₂)_(p) (in which 1≦p≦10), (CF₂)_(q) (in which 1≦q≦10), C(CH₃)₂, C(CF₃)₂ and C(═O)NH; Q is a single bond, O, S, C(═O), CH(OH), S(═O)₂, Si(CH₃)₂, (CH₂)_(p) (in which 1≦p≦10), (CF₂)_(q) (in which 1≦q≦10), C(CH₃)₂, C(CF₃)₂, C(═O)NH, C(CH₃)(CF₃), C₁-C₆ alkyl-substituted phenyl or C₁-C₆ haloalkyl-substituted phenyl in which Q is linked to opposite both phenyl rings in the position of m-m, m-p, p-m or p-p; and n is an integer of 20 to 400.

In still yet another aspect, the present invention provides a gas separation membrane comprising polybenzoxazole (TR-β-PBO) represented by the formula:

wherein Ar is a bivalent C₅-C₂₄ arylene group or a bivalent C₅-C₂₄ heterocyclic ring, which is substituted or unsubstituted with at least one substituent selected from the group consisting of C₁-C₁₀ alkyl, C₁-C₁₀ alkoxy, C₁-C₁₀ haloalkyl and C₁-C₁₀ haloalkoxy, or two or more of which are fused together to form a condensation ring, or covalently bonded to each other via a functional group selected from the group consisting of O, S, C(═O), CH(OH), S(═O)₂, Si(CH₃)₂, (CH₂)_(p) (in which 1≦p≦10), (CF₂)_(q) (in which 1≦q≦10), C(CH₃)₂, C(CF₃)₂ and C(═O)NH; Q is a single bond, O, S, C(═O), CH(OH), S(═O)₂, Si(CH₃)₂, (CH₂)_(p) (in which 1≦p≦10), (CF₂)_(q) (in which 1≦q≦10), C(CH₃)₂, C(CF₃)₂, C(═O)NH, C(CH₃)(CF₃), C₁-C₆ alkyl-substituted phenyl or C₁-C₆ haloalkyl-substituted phenyl in which Q is linked to opposite both phenyl rings in the position of m-m, m-p, p-m or p-p; and n is an integer of 20 to 400, wherein the polybenzoxazole has a weight average molecular weight of from over about 50,000 Da to about 300,000 Da.

DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is graphs showing TGA-MS results of the PHA precursor membrane of Example 1 and the HPI precursor membrane of Comparative Example 1;

FIG. 2( a) is FT-IR spectra of the HPI precursor membrane and the TR-α-PBO membrane of Comparative Example 1 and FIG. 2( b) is FT-IR spectra of the PHA precursor membrane and TR-β-PBO membrane of Example 1;

FIG. 3 is DSC thermograms of the PHA precursor membrane and the TR-β-PBO membrane Example 1 and the HPI precursor membrane and the TR-α-PBO membrane of Comparative Example 1;

FIG. 4( a) is X-ray diffraction patterns of the HPI precursor membrane and the TR-α-PBO membrane of Comparative Example 1 and FIG. 4( b) is X-ray diffraction patterns of the PHA precursor membrane and the TR-β-PBO membrane of Example 1;

FIG. 5( a) is adsorption isotherms of constant-pressure simulations for O₂ and FIG. 5(b) is adsorption isotherms of constant-pressure simulations for N₂;

FIG. 6 is N₂ adsorption/desorption isotherms at −196° C. for the HPI precursor membrane (a) and the TR-α-PBO membrane (b) of Comparative Example 1, and the PHA precursor membrane (c) and TR-β-PBO membrane (d) of Example 1;

FIG. 7( a) is a graph showing H₂ permeability-H₂/N₂ selectivity of the TR-β-PBO membrane and conventional polymer membranes and FIG. 7( b) is a graph showing H₂ permeability-H₂/CH₄ selectivity of the TR-β-PBO membrane and conventional polymer membranes;

FIG. 8 illustrates IR spectra of certain PHAs according to the present invention;

FIGS. 9( a), (b), and (c) illustrate the thermogravimetric analysis with trace of m/e 18 (H₂O) (TG-MS) of certain PHAs;

FIG. 10 illustrates IR spectra of certain TR-β-PBOs according to the present invention;

FIGS. 11( a), (b), and (c) illustrate the cavity diameters and intensities measured by positron annihilation lifetime spectroscopy (PALS) for (a) 6FIP, (b) 6FTP and (c) 6F6F membranes treated from 250 to 350° C.;

FIGS. 12( a) and (b) illustrate wide-angle X-ray diffraction (WAXD) patterns of certain PHAs and TR-β-PBOs according to the present invention;

FIGS. 13( a), (b), and (c) illustrate the trade-off relationships in PHAs and TR-β-PBOs for (a) O₂/N₂, (b) CO₂/CH₄ and (c) H₂/CO₂ at 300K;

FIGS. 14( a) and (b) illustrate the temperature dependence on cavity diameters and intensities for 5a membrane by PALS (a) τ3 and (b) τ4; and

FIGS. 15( a) and (b) illustrate the temperature dependence on H₂ permeability and H₂/CO₂ selectivity of TR-β-PBO membranes at 20 bar (a) with temperature variation (filled: H₂ permeability, empty: H₂/CO₂ selectivity), (b) with polymeric upper bounds.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS THE INVENTION

Hereinafter, the present invention will be illustrated in more detail.

The preparation method of the present invention comprises thermally converting poly(hydroxyamide) into polybenzoxazole through thermal treatment involving dehydration.

Specifically, the poly(hydroxyamide) represented by Formula 2 as a precursor is converted into the benzoxazole-based polymer represented by Formula 1, as depicted in Reaction Scheme 1 below:

wherein Ar is a bivalent C₅-C₂₄ arylene group or a bivalent C₅-C₂₄ heterocyclic ring, which is substituted or unsubstituted with at least one substituent selected from the group consisting of C₁-C₁₀ alkyl, C₁-C₁₀ alkoxy, C₁-C₁₀ haloalkyl and C₁-C₁₀ haloalkoxy, or two or more of which are fused together to form a condensation ring, or covalently bonded to each other via a functional group selected from the group consisting of O, S, C(═O), CH(OH), S(═O)₂, Si(CH₃)₂, (CH₂)_(p) (in which 1≦p≦10), (CF₂)_(q) (in which 1≦q≦10), C(CH₃)₂, C(CF₃)₂ and C(═O)NH;

Q is a single bond, O, S, C(═O), CH(OH), S(═O)₂, Si(CH₃)₂, (CH₂)_(p) (in which 1≦p≦10), (CF₂)_(q) (in which 1≦q≦10), C(CH₃)₂, C(CF₃)₂, C(═O)NH, C(CH₃)(CF₃), C₁-C₆ alkyl-substituted phenyl or C₁-C₆ haloalkyl-substituted phenyl in which Q is linked to opposite both phenyl rings in the position of m-m, m-p, p-m or p-p; and

n is an integer of 20 to 400.

Preferably, Ar is selected from the following compounds and the linkage position thereof includes all of o-, m- and p-positions.

wherein X is O, S, C(═O), CH(OH), S(═O)₂, Si(CH₃)₂, (CH₂)_(p) (in which 1≦p≦10), (CF₂)_(q) (in which 1≦q≦10), C(CH₃)₂, C(CF₃)₂, or C(═O)NH; Y is O, S or C(═O); and Z₁, Z₂ and Z₃ are identical or different and are O, N or S.

More preferably, Ar is selected from the following compounds:

In Formula 1, Q is a single bond, C(CH₃)₂, C(CF₃)₂, C(═O)NH, C(CH₃)(CF₃),

More preferably, Ar is

and Q is C(CF₃)₂.

As can be seen from Reaction Scheme 1, the poly(hydroxyamide) 2 as a precursor is converted into the benzoxazole-based polymer 1. The conversion of the poly(hydroxyamide) 2 into the benzoxazole-based polymer 1 is carried out by dehydration, namely, removal of H₂O present in the poly(hydroxyamide) 2.

After the thermal rearrangement through the thermal treatment, the benzoxazole-based polymer 1 undergoes morphological changes including reduced density, considerably increased fractional free volume (FFV) due to increased microcavity size and increased d-spacing, as compared to the precursor 2. As a result, the benzoxazole-based polymer 1 exhibits considerably high gas permeability, as compared to the precursor 2. In addition, the benzoxazole-based polymer 1 exhibits improved tensile strength and elongation.

These morphological properties can be readily controlled by a design taking into consideration the characteristics (e.g., steric hindrance) of Ar and Q, the functional groups present in the molecular structures, and permeability and selectivity for various types of gases can be thus controlled.

According to the present invention, the thermal treatment is carried out at 150 to 450° C., preferably 250 to 350° C., at a heating rate of 1 to 10° C./min for 5 minutes to 12 hours, preferably for 10 minutes to 2 hours, under an inert atmosphere. When the thermal treatment temperature is less than the level as defined the above, the thermal rearrangement is incomplete, thus leaving precursor residues, causing deterioration of purity. Increasing the thermal treatment temperature above the level defined above provides no particular advantage, thus being economically impractical. Accordingly, the thermal treatment is properly carried out within the temperature range as defined above.

At this time, the reaction conditions are properly controlled according to Ar and Q, the functional groups of the precursor, and specific conditions can be adequately selected and modified by those skilled in the art.

Preferably, the benzoxazole-based polymer 1 is designed in the preparation process such that it has a desired molecular weight. Preferably, the weight average molecular weight of the benzoxazole-based polymer 1 is adjusted to from 10,000 to 300,000 Da. When the weight average molecular weight is less than 10,000 Da, physical properties of the polymer are poor. In certain preferred embodiments of the present invention, the weight average molecular weight of the benzoxazole-based polymer 1 is preferably from more than about 50,000 Da to about 300,000 Da, and more preferably from more than about 50,000 Da to about 200,000 Da. Benefits of employing such higher molecular weight benzoxazole-based polymers as gas separation membranes according to the present invention include, relative to the lower molecular weight polymers, increased tensile strength, toughness, flexural strength, chemical resistance, and thermal resistance.

In particular, the poly(hydroxyamide) 2 used as a precursor in the present invention is prepared by a conventional method.

For example, the poly(hydroxyamide) 2 is prepared by reacting the compound 4 with the compound 5, as depicted in Reaction Scheme 2 below:

wherein X is a halogen atom, and Ar, Q and n are defined as above.

Preferably, the halogen atom is F, Cl, Br or I. More preferred is the use of Cl in view of its high reactivity.

For example, terephthaloyl chloride (TCL) and 2,2′-bis(3-amino-4-hydroxyphenyl) hexafluoropropane (bisAPAF) are used as the compounds of Formulae 4 and 5, respectively.

The compounds 4 and 5 are suitably selected in conformity with Ar and Q defined throughout the present specification. Taking stoichiometry into consideration, the compounds 4 and 5 are used in a desired molar ratio, preferably, in the range of 1:1 to 2:1, and more preferably, an excess of the compound 4 is used.

The reaction is carried out at −10 to 60° C. for 30 minutes to 12 hours until the reaction is fully completed.

Furthermore, an acid acceptor is added to capture HX (hydrogen halide, i.e. HCl) produced during the reaction. The acid acceptor is selected from the group consisting of ethylene oxide, propylene oxide, magnesium oxide, hydrotalcite, magnesium carbonate, calcium hydroxide, magnesium silicate and combinations thereof. Preferably, an excess of the acid acceptor is used, as compared to HX, the reaction product.

The benzoxazole-based polymer 1 prepared by the method of the present invention as mentioned above is suited for application to gas separation membranes due to superior gas permeability and selectivity thereof.

The present invention is not limited to the preparation method of the gas separation membrane. That is, the gas separation membrane can be prepared in the form of films or fibers (in particular, hollow fibers) by a conventional method e.g. casting or laminating.

For example, the gas separation membrane made of the benzoxazole-based polymer 1 is prepared by casting the precursor 2 onto a substrate, followed by thermal treatment, as depicted in Reaction Scheme 1.

The benzoxazole-based polymer-comprising gas separation membrane according to the present invention is prepared by preparing a polymer precursor and subjecting the precursor to thermal conversion involving dehydration. Accordingly, in terms of physical properties, the polybenzoxazole gas separation membrane according to the present invention is remarkably different from gas separation membranes made of polybenzoxazole (TR-α-PBO), which is prepared by preparing a conventional polymer precursor and subjecting the precursor to thermal treatment involving removal of CO₂.

First, glass transition temperatures (Tg, 400° C. or higher) of conventional polymers prepared through CO₂ removal are impossible to measure due to a rigid structure thereof, while Tg of the polybenzoxazole of the present invention is measured to be 377° C. (in the case of polybenzoxazole prepared in Example 1) due to its soft molecular structure, thus being preferably applicable to gas separation membranes.

Second, the gas separation membrane of the present invention is useful for gas separation membranes due to high tensile strength and elongation thereof (See Table 2).

Third, in terms of morphological properties, the gas separation membrane has well-connected microcavities and exhibits a superior fractional free volume, allowing gases to smoothly pass though the microcavities (good permeability).

Fourth, the gas separation membrane has a low d-spacing, thus exhibiting increased permselectivity for small gases.

Fifth, the gas separation membrane is useful as a gas separation membrane for gas pair such as H₂/CH₄, H₂/CO₂, H₂/N₂, He/N₂, O₂/N₂, CO₂/N₂, and CO₂/CH₄, preferably, as a gas separation membrane applicable to gas pair such as H₂/CH₄, H₂/CO₂, H₂/N₂ and He/N₂, including small gases such as H₂ or He. These gas separation membranes have high selectivity for small gases due to their polymeric microcavities.

Sixth, the benzoxazole-based polymer according to the present invention can be designed by modifying functional groups in the molecular structure thereof, thus being used to prepare various gas separation membrane products.

In a preferred embodiment of the present invention, the polybenzoxazole polymer is polybenzoxazole (TR-β-PBO) represented by Formula 3 below:

The polybenzoxazole (TR-β-PBO, 3) is prepared by thermally treating the poly(hydroxyamide) (PHA, 8), as depicted in Reaction Scheme 3 below:

The thermal treatment is carried out at 150 to 400° C., preferably 250 to 350° C., at a heating rate of 1 to 10° C./min, for 30 minutes to 12 hours, preferably for 30 minutes to 2 hours, under an inert atmosphere.

The precursor poly(hydroxyamide) (PHA, 8) is prepared by reacting terephthaloyl chloride (TCL, 6) with 2,2′-bis(3-amino-4-hydroxyphenyl) hexafluoropropane (bisAPAF, 7), as depicted in Reaction Scheme 4 below:

The reaction is carried out at −10 to 60° C. for 30 minutes to 12 hours until the reaction is thoroughly completed.

In addition, an acid acceptor is added to capture HX (hydrogen halide, e.g., HCl) produced during the reaction. The acid acceptor is selected from the group consisting of ethylene oxide, propylene oxide, magnesium oxide, hydrotalcite, magnesium carbonate, calcium hydroxide, magnesium silicate and combinations thereof. Preferably, an excess of the acid acceptor is used, as compared to HX, the reaction product.

The polybenzoxazole (TR-β-PBO, 3) prepared by thermal treatment as mentioned above has a glass transition temperature (Tg) of 377°, a d-spacing of 6.0 to 6.10 Å and a rigid rod-type structure.

The polybenzoxazole (TR-β-PBO, 3) of the present invention is prepared from the poly(hydroxyamide) precursor and thus has mechanical and morphological properties different from conventional polybenzoxazole (conventionally known as TR-α-PBO) (See Table 2).

That is to say, the TR-α-PBO is prepared by thermally treating polyimide as a precursor. Tg of the TR-α-PBO is impossible to measure. On the other hand, Tg of the TR-β-PBO of the present invention is observed at 377° C., as mentioned above. The observable Tg means that the TR-β-PBO has soft polymeric chains, which affects mechanical properties such as tensile strength and elongation.

Furthermore, the TR-β-PBO has a superior fractional free volume (FFV) property and a d-spacing of 6.0 to 6.10 Å, preferably 6.02 Å, which is different from the d-spacing (i.e., 6.25 Å) of TR-α-PBO. The difference in d-spacing affects gas permeability and selectivity when used for gas separation membranes.

Consequently, the conventional TR-α-PBO and the present TR-β-PBO have identical repeating units, but have different physical properties, thus providing greatly different effects when used for gas separation membranes. This is achieved by thermally treating the present precursor in the range of specific temperatures. Preferably, the thermal treatment is carried out at 150 to 450° C., preferably 250 to 350° C., at a heating rate of 1 to 10° C./min, for 5 minutes to 12 hours, preferably for 10 minutes to 2 hours, under an inert atmosphere. When the temperature is less than the level as defined the above, thermal rearrangement does not proceed to completion, thus leaving precursor residues, which reduces purity. Exceeding the temperature as defined above provides no significant advantage and is this economically disadvantageous. Accordingly, the thermal treatment is properly carried out within the temperature range as defined above.

In particular, the gas separation membrane comprising the TR-β-PBO of Formula 3 is prepared by a conventional method. In one embodiment, the method comprises reacting terephthaloyl chloride (TCL) with 2,2′-bis(3-amino-4-hydroxyphenyl) hexafluoropropane (bisAPAF) to prepare poly(hydroxyamide) (PHA); casting the poly(hydroxyamide) (PHA) on a substrate, followed by drying, to prepare a precursor membrane; and thermally treating the precursor membrane.

The drying is carried out at 50 to 200° C. for 30 minutes to 5 hours. The thermal treatment is carried out at 150 to 450° C., preferably at 250 to 350° C., at a heating rate of 1 to 10° C./min, for 5 minutes to 12 hours, preferably for 10 minutes to 2 hours under an inert atmosphere.

The TR-β-PBO gas separation membrane thus prepared exhibits superior physical properties (e.g., tensile strength of 85 to 90 MPa and elongation of 5 to 10%).

The TR-β-PBO gas separation membrane is useful as a gas separation membrane applicable to gas pair such as H₂/CH₄, H₂/CO₂, H₂/N₂, He/N₂, O₂/N₂, CO₂/N₂, and CO₂/CH₄, preferably, as gas separation membranes applicable to gas pairs such as H₂/CH₄, H₂/CO₂, H₂/N₂, and He/N₂, including small gases such as H₂ or He. Due to polymeric microporous properties thereof, the TR-β-PBO gas separation membrane has high selectivity for small gas series, which cannot be realized by conventional TR-s separation membrane ables 6 and 7).

Moreover, the TR-β-PBO gas separation membranes according to the present invention are useful as a gas separation membrane for high temperature separations applicable to gas pairs such as H₂/CH₄, H₂/CO₂, H₂/N₂, He/N₂, O₂/N₂, CO₂/N₂, and CO₂/CH₄, preferably, as high temperature gas separation membranes applicable to gas pair such as H₂/CH₄, H₂/CO₂, H₂/N₂, and He/N₂, including small gases such as H₂ or He. As used herein, the term “high temperature” as it relates to gas separation refers to temperatures of from about 30° C. to about 400° C., and preferably from about 200° C. to about 350° C.

TR-betα-PBO gas separation membranes according to the present invention can take the physical form of various configurations well known in the art such as, for example, the form of hollow fibers, films, tubular shapes, as flat sheets in spiral wound configurations, or plate and frame configurations. The following examples demonstrate certain embodiments of high temperature gas separation according to the present invention.

EXAMPLES

Hereinafter, preferred examples will be provided for a further understanding of the invention. These examples are for illustrative purposes only and are not intended to limit the scope of the present invention.

Example 1 Preparation of polybenzoxazole (TR-β-PBO) separation membrane

TR-β-PBO represented by Formula 3 below was prepared through the following reaction.

2,2′-bis(3-amino-4-hydroxyphenyl) hexafluoropropane (bisAPAF, 3.663 g, 10 mmol) and NMP (15.06 mL) were charged into a 100 mL 3-neck flask under nitrogen purging and the mixture was placed into an ice bath at 0° C. Subsequently, a solution of propylene oxide (PO, 0.3 mL) and terephthaloyl chloride (TCL, 2.030 g, 10 mmol) in NMP (8.35 mL) was added to the mixture and then allowed to proceed for 2 hours.

The resulting mixture was stirred for 12 hours under an inert atmosphere to obtain a viscous poly(hydroxyamide) (PHA) solution.

The solution was cast onto a glass substrate and dried at 100° C. for one hour and at 200° C. for 10 hours to remove the solvent, thereby obtaining a PHA precursor membrane.

The PHA precursor membrane was thermally treated at 350° C. at a heating rate of 5° C./min for one hour under an Ar atmosphere and was then allowed to slowly cool to ambient temperature to prepare a polybenzoxazole (TR-β-PBO) separation membrane.

Comparative Example 1 Preparation of Polybenzoxazole (Tr-α-PBO) Separation Membrane

TR-α-PBO was prepared in accordance with the following Reaction Scheme 5.

BisAPAF (3.663 g, 10 mmol) and NMP (21.34 mL) were charged into a 100 mL 3-neck flask under nitrogen purging. A solution of 1,2,4,5-benzenetetracarboxylic dianhydride (PMDA, 2.181 g, 10 mmol) in NMP (12.71 mL) was added thereto. The mixture was allowed to react at ambient temperature for 5 hours to obtain a viscous yellow solution. The reaction was allowed to proceed for an additional 12 hours to obtain a polyamic acid (PAA) solution.

The polyamic acid (PAA) solution was cast onto a glass substrate and then thermally treated at 100° C. for one hour and at 300° C. for one hour under reduced pressure to remove the solvent, thereby obtaining a hydroxy-containing polyimide (HPI) precursor membrane.

The HPI precursor membrane was thermally treated at 450° C. with a heating rate of 5° C./min for one hour under an Ar atmosphere and was then allowed to slowly cool to ambient temperature to obtain a polybenzoxazole (TR-α-PBO) separation membrane.

TABLE 1 Example 1 Comparative Example 1 Heating 350° C., 1 hour 450° C., 1 hour conditions Intermediate

  PHA

  HPI Finally produced PBO

  TR-β-PBO

  TR-α-PBO

The physical properties were evaluated for TR-β-PBO and TR-α-PBO separation membranes prepared in Example 1 and Comparative Example 1 and precursor membranes thereof.

Experimental Example 1 Thermogravimetric Analysis/Mass Spectroscopy (TGA-MS)

The PHA precursor membrane of Example 1 and the HPI precursor membrane of Comparative Example 1 were subjected to TGA-MS to confirm dehydration and CO₂ evolution. The TGA-MS for each precursor membrane was carried out using TG 209 F1 Iris and QMS 403C Aeolos (NETZSCH, Germany). The results thus obtained are shown in FIG. 1.

FIG. 1 is a graph showing TGA-MS results of the PHA precursor membrane of Example 1 and the HPI precursor membrane of Comparative Example 1.

As can be confirmed from FIG. 1, the PHA precursor membrane of Example 1 undergoes weight loss at 250 to 350° C. (represented by reference numeral a′ in FIG. 1) corresponding to the temperature at which thermal conversion from PHA to TR-β-PBO occurs, and MS peaks indicating dehydration (removal of H₂O) are plotted at 300° C. (represented by reference numeral b in FIG. 1). On the other hand, it can be confirmed from FIG. 1 that the HPI precursor membrane of Comparative Example 1 undergoes weight loss at 350 to 450° C. (represented by reference numeral b in FIG. 1) corresponding to the temperature at which thermal conversion from PHA to TR-β-PBO occurs, and MS peaks indicating evolution of CO₂ are plotted at about 450° C. (represented by reference numeral c in FIG. 1).

These TGA-MS results show that all TR-α-PBO and TR-β-PBO membranes are thermally stable up to a maximum 500° C.

Experimental Example 2 FT-IR analysis

The PHA precursor membrane and TR-β-PBO membrane of Example 1, and HPI precursor membrane and TR-α-PBO membrane of Comparative Example 1 were subjected to FT-IR analysis to confirm characteristic peaks. FT-IR spectra were obtained using a Nicolet Magna IR 860 instrument (thermo Nicolet, Madison, Wis., USA). The results thus obtained are shown in FIGS. 2( a) and 2(b).

FIG. 2( a) is FT-IR spectra of the HPI precursor membrane and the TR-α-PBO membrane of Comparative Example 1. FIG. 2( b) is FT-IR spectra of the PHA precursor membrane and TR-β-PBO membrane of Example 1.

As can be seen from FIGS. 2( a) and 2(b), broad bands (a and f) by O—H stretching of HPI and PHA are observed at 3,700 to 2,500 cm⁻¹.

As apparent from FIG. 2( a), the HPI precursor membrane shows characteristic absorption bands of imide groups at 1,729 cm⁻¹ (C═O stretching, c) and 1,781 cm⁻¹ (C═O stretching, b), and as apparent from FIG. 2( b), the PHA precursor membrane shows characteristic absorption peaks of amide groups at 1,650 cm⁻¹ (C═O stretching, g) and 1,530 cm⁻¹ (N—H bending, h).

In addition, after thermal conversion into PBO, all of the TR-α-PBO and TR-β-PBO membranes show peaks corresponding to benzoxazole rings at 1,058 cm⁻¹ (C—O stretching, e, j), 1,480 cm⁻¹ and 1,558 cm⁻¹ (C═N stretching, d, i).

Experimental Example 3 Element Analysis

The PHA precursor membrane and TR-β-PBO membrane of Example 1, and the HPI precursor membrane and the TR-α-PBO membrane of Comparative Example 1 were subjected to element analysis (EA) to confirm elements present in the membrane. The element analysis was carried out using an elemental analyzer (Flash EA 1112, CE Instruments, UK). The results thus obtained are shown in Table 2 below.

TABLE 2 Type Formula C (wt. %) H (wt. %) N (wt. %) Exam. 1 PHA precursor membrane [C₂₃H₁₄F₆N₂O₄]_(n) 54.06 ± 0.10 2.73 ± 0.12 5.75 ± 0.13 (55.7)* (2.84)* (5.64)* TR-β-PBO membrane [C₂₃H₁₀F₆N₂O₂]_(n) 60.33 ± 0.04 2.15 ± 0.08 6.01 ± 0.05 (60.0)* (2.19)* (6.09)* Comp. HPI precursor membrane [C₂₃H₁₀F₆N₂O₆]_(n)  53.3 ± 0.04 1.91 ± 0.04 4.92 ± 0.02 Exam. 1 (54.8)* (1.84)* (5.11)* TR-α-PBO membrane [C₂₃H₁₀F₆N₂O₂]_(n) 60.52 ± 0.05 2.05 ± 0.06 6.14 ± 0.07 (60.0)* (2.19)* (6.09)* *Theoretical values

Experimental Example 4 Differential Scanning Calorimetry (DSC) Analysis

The PHA precursor membrane and the TR-β-PBO membrane of Example 1 and the HPI precursor membrane and the TR-α-PBO membrane of Comparative Example 1 were subjected to DSC analysis to measure glass transition temperatures (Tg) thereof. The DSC analysis was carried out using a DSC-2010 TA Instruments system at a heating rate of 20° C./min under an N₂ atmosphere. The results thus obtained are shown in FIG. 3.

FIG. 3 is DSC thermograms of PHA precursor membrane and TR-β-PBO membrane Example 1 and the HPI precursor membrane and TR-α-PBO membrane of Comparative Example 1.

As can be seen from FIG. 3, Tg of the PHA precursor membrane and the TR-β-PBO membrane were observed at 281° C. and 377° C., respectively. This behavior is attributed to the rigid rod structure of benzoxazole. In addition, Tg of the HPI precursor membrane was observed at 353° C. However, Tg of TR-α-PBO membrane obtained therefrom cannot be measured.

These results indicated that TR-β-PBO membrane chains are softer and more flexible than TR-α-PBO membrane chains.

Experimental Example 5 Analysis of Tensile Strength and Elongation

The tensile strength and elongation were measured for the PHA precursor membrane and TR-β-PBO membrane of Example 1 and HPI precursor membrane and TR-α-PBO membrane of Comparative Example 1. For measurement of the physical properties, five specimens for respective membranes with a width of 0.5 cm, a length of 4 cm and a thickness of 60-70 μm were prepared. The physical properties were characterized to study stress-strain behavior of the polymer samples using an Autograph AGS-J (Shimadzu, Kyoto, Japan). The results thus obtained are shown in Table 3 below:

TABLE 3 Tensile strength Elongation Type (MPa) (%) Ex. 1 PHA precursor membrane 63 2.3 TR-β-PBO precursor membrane 87 6.0 Comp. HPI precursor membrane 62 2.7 Ex. 1 TR-α-PBO precursor membrane 69 3.4

As can be seen from Table 3 above, the polybenzoxazole membrane shows increased tensile strength and elongation, as compared to precursor membranes. In particular, the TR-β-PBO membrane according to Example 1 of the present invention has even higher tensile strength and elongation than the TR-α-PBO membrane of Comparative Example 1. This means that the membranes prepared by the method according to the present invention are more flexible and have higher strength.

Experimental Example 6 Wide Angle X-ray Diffraction Pattern Analysis

The PHA precursor membrane and the TR-β-PBO membrane of Example 1 and the HPI precursor membrane and the TR-α-PBO membrane of Comparative Example 1 were subjected to wide-angle X-ray diffraction (WAXD) analysis to confirm morphologies thereof. The analysis was carried out using a wide angle X-ray diffractometer (D/MAX-2500, Rigaku, Japan).

FIG. 4( a) is X-ray diffraction patterns of the HPI precursor membrane and the TR-α-PBO membrane of Comparative Example 1. FIG. 4( b) is X-ray diffraction patterns of the PHA precursor membrane and the TR-β-PBO membrane of Example 1.

As can be seen from FIG. 4, all of the membranes show broad patterns, meaning that they have an amorphous structure. In addition, after the thermal conversion from the HPI precursor membrane to the TR-α-PBO membrane, the peak center (2θ) shifts from 14.6 to 14.15 degrees, and after thermal conversion from the PHA precursor membrane to the TR-β-PBO membrane, the peak center (2θ) shifts from 15.4 to 14.7 degrees.

Experimental Example 7 Measurement of Free Volume-Related Physical Properties

The physical properties were measured for the PHA precursor membrane, TR-β-PBO membrane of Example 1 and the HPI precursor membrane and the TR-α-PBO membrane of Comparative Example 1. The results thus obtained are shown in Table 4 below.

First, the density of the membranes was measured by a buoyancy method using a Sartorius LA 120S analytical balance. The fractional free volume (FFV, Vf) was calculated from the data in accordance with Equation 1 below [W. M. Lee. Selection of barrier materials from molecular structure. Polym Eng Sci. 1980, 20, 65-9].

$\begin{matrix} {{FFV} = \frac{V - {1.3\; {Vw}}}{V}} & {{Equation}\mspace{14mu} 1} \end{matrix}$

wherein V is the polymer specific volume and V_(w) is the specific Van der Waals volume. The Van der Waals volume was estimated by a Cerius 4.2 program using a synthia module based on the work of J. Bicerano [J. Bicerano. Prediction of polymer properties, Third Edition. Marcel Dekker Inc. 2002].

The d-spacing was calculated in accordance with Bragg's equation from X-ray diffraction pattern results.

TABLE 4 V V_(wb) Incre- D- Density (cm³/ (cm³/ V_(f) ment in spacing Type (g/cm³) g) g) (FFV) V_(f) (%) (Å) Ex. 1 PHA 1.450 0.690 0.462 0.129 5.75 precursor membrane TR-β- 1.413 0.708 0.444 0.184 +43 6.02 PBO membrane Comp. HPI 1.478 0.667 0.443 0.148 6.06 Ex. 1 precursor membrane TR-α- 1.362 0.734 0.457 0.190 +28 6.25 PBO membrane b: value measured with MS modeling software 4.0

As can be seen from Table 4, in the case of Comparative Example 1, the density of the thermally converted TR-α-PBO membrane was considerably lower than that of the HPI precursor membrane due to the evolution of CO₂ generated during thermal conversion, and in the case of Example 1, the density of the thermally converted TR-β-PBO membrane was slightly lower than that of the PHA precursor membrane due to dehydration during thermal conversion.

Furthermore, V_(f) of thermally converted PBOs is higher than those of respective precursors due to thermal rearrangement in a solid state. The TR-β-PBO membrane shows a slightly lower V_(f) than the TR-α-PBO membrane, but there is no significant difference in V_(f) between the membranes.

As can be seen from Table 4, the d-spacing of the PHA precursor membrane and the TR-β-PBO membrane are substantially lower than those of the HPI precursor membrane and TR-α-PBO membrane. The decrease in d-spacing affects pores and free volume elements, allowing permeation of smaller gas molecules.

Experimental Example 8 Molecular Dynamics (MD) Simulation of Gas Sorption

The PHA and HPI precursor membranes and PBO polymer membrane were simulated using the computer program Materials Studio modeling to confirm gas adsorption properties. The 4.2 COMPASS force field (Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies) was used in all the simulations. Molecular dynamics (MD) were calculated using the Amorphous Cell module of the MS program. The O₂ and N₂ sorption amounts were also calculated from a sorption module allowing simulation of absorption of pure sorbate. Adsorption isotherms obtained from constant-pressure simulations for O₂ and N₂ are shown in FIG. 5.

FIG. 5( a) is adsorption isotherms of constant-pressure simulations for O₂. FIG. 5( b) is adsorption isotherms of constant-pressure simulations for N₂.

As can be seen from FIGS. 5( a) and 5(b), in the case of the HPI and PHA precursor membranes, average O₂ and N₂ loading per cell was about 10 to 1,000 kPa due to their low fractional free volume contents. In contrast, the PBO membrane showed extremely high gas loadings around low fugacity region. These results, obtained from molecular simulations, indicate that the PBO separation membranes can sufficiently accumulate gas molecules therein.

Experimental Example 9 Nitrogen Adsorption and Desorption Analysis

The PHA precursor membrane and the TR-β-PBO membrane of Example 1 and the HPI precursor membrane and the TR-α-PBO membrane of Comparative Example 1 were subjected to N₂ adsorption/desorption experiments. The BET adsorption isotherms for N₂ at 77K were determined using a Micrometrics ASAP 2020 surface area and porosity analyzer (Atlanta, USA). The adsorbents were degassed at 200° C. overnight before the adsorption measurements. The specific surface areas, S_(BET), were calculated from the linear form of the Brunauer-Emmett-Teller (BET) equation.

FIG. 6 is N₂ adsorption/desorption isotherms at −196° C. for the HPI precursor membrane (a) and the TR-α-PBO membrane (b) of Comparative Example 1, and the PHA precursor membrane (c) and the TR-β-PBO membrane (d) of Example 1.

As can be seen from FIG. 6, all the TR-α-PBO and TR-β-PBO membranes show a higher nitrogen volume than those of their precursor membranes. This means that the thermally treated PBO membranes have increased pore size, as compared to precursor membranes.

As mentioned above, the TR-PBO membranes induced by the precursors, HPI and PHA, have larger microcavities than those of the precursors. In particular, as apparent from Tables 1 to 3 and FIGS. 1 to 6, there are differences in properties between TR-β-PBO using PHA as the precursor and TR-α-PBO using HPI as the precursor. Furthermore, the TR-β-PBO membranes have a lower d-spacing than the TR-α-PBO membranes, thus enabling efficient separation of gas pair including small gases.

Experimental Example 10 Gas Permeability and Permselectivity Analysis

For the PHA precursor membrane and TR-β-PBO membrane of Example 1 and the HPI precursor membrane and the TR-α-PBO membrane of Comparative Example 1, permeability and permselectivity for various gases were measured.

The gas permeability was measured with high vacuum time-lag equipment using single gases (1 bar, 25° C.). Five samples with a thickness 30 μm for respective membranes were used. The results thus obtained are shown in Table 5 below.

TABLE 5 Gas permeability Gas permeability O₂ N₂ CO₂ H₂ He CH₄ (size) (Barrer^(a)) (3.46 Å) (3.64 Å) (3.36 Å) (2.89 Å) (2.6 Å) (3.80 Å) Ex. 1 PHA precursor 1 0.2 4 15 24 0.1 membrane TR-β-PBO membrane 15 3 58 114 121 2 Comp. HPI precursor membrane 4 1 17 43 62 0.2 Ex. 1 TR-α-PBO membrane 148 34 952 635 421 23 (Barrer^(a)): 10⁻¹⁰ (cm³⁽STP)cm/cm²scmHg)

TABLE 6 Gas selectivity O₂/ H₂/ CO₂/ He/ H₂/ CO₂/ Type N₂ N₂ N₂ N₂ CH₄ CH₄ Ex. 1 PHA precursor 6 89 25 136 186 52 membrane TR-β-PBO membrane 5 39 20 42 58 30 Comp. HPI precursor membrane 7 78 31 113 200 80 Ex. 1 TR-α-PBO membrane 4 19 28 13 27 41

As can be seen from Table 5 above, gas permeabilities of the TR-β-PBO and TR-α-PBO membranes were significantly higher than those of the precursor membranes.

As apparent from Table 6, when H₂/CH₄, H₂/N₂, He/N₂ and O₂/N₂ are separated, in terms of selectivity, the precursor membranes are superior to PBO membranes, but the TR-β-PBO membrane is still higher than TR-α-PBO membrane.

Useful separation membranes must be selected, taking into consideration the permeability and selectivity. In this regard, the TR-β-PBO membrane prepared according to the present invention exhibits superior permeability and selectivity, and in particular is more effective in separating small gases such as H₂ and He.

Experimental Example 11 Hydrogen Mix Gas Permeability Analysis

The hydrogen permeability and selectivity of the TR-β-PBO membrane according to the present invention and conventional polymer separation membranes were measured at 30° C. The results thus obtained are shown in Table 7 below:

TABLE 7 H₂ permea- bility Selectivity Polymer (Barrers^(b)) H₂/N₂ H₂/CH₄ H₂/CO₂ H₂/CO TR-β-PBO 114 39 58 2 37 TR-α-PBO 635 19 27 0.7 12 Celluose acetate 3 12.5 12.4 0.4 — Ethyl cellulose 87 27.2 4.6 3.3 — Polybenzimidazole 0.09 — — 9 — Polyetherimide 8 166 222.9 5.9 — Polydimethylsiloxane 375 1.3 0.6 0.3 — Polyimide(Matrimid) 28 87.8 112.4 2.6 — Polymethylmetacrylate 2 2 4 4 — Polymethylpentene 125 18.7 8.4 1.5 — Polyphenyleneoxide 113 29.7 10.3 1.5 — Polystyrene 24 39.7 29.8 2.3 12 Polysulfone 12 15.1 30.3 2 38 Polyvinyl acetate 15 11.6 16.8 1.2 — (Barrer^(a)): 10⁻¹⁰ (cm³⁽STP)cm/cm²scmHg)

As can be seen from Table 7, the TR-β-PBO polymers according to the present invention exhibit superior hydrogen permeability and selectivity for gas pair, as compared to other polymers, thus being useful for separation membranes.

Experimental Example 12 Analysis of Correlation Between Permeability and Selectivity

The H₂ permeability and selectivity for H₂/N₂ and H₂/CH₄ of the TR-β-PBO membrane according to the present invention and conventional polymer membrane were measured. The results thus obtained are shown in FIGS. 7( a) and 7(b).

FIG. 7( a) is a graph showing H₂ permeability-H₂/N₂ selectivity of the TR-β-PBO membrane and conventional polymer membranes and FIG. 7( b) is a graph showing H₂ permeability-H₂/CH₄ selectivity of the TR-β-PBO membrane and conventional polymer membranes.

As can be seen from FIGS. 7( a) and 7(b), the TR-β-PBO membrane according to the present invention exhibits superior permeability and selectivity, as compared to conventional polymer membranes.

High Temperature Separation Performance of High Molecular Weight Polybenzoxazoles

Materials. To prepare thermally rearranged polybenzoxazoles for high temperature gas separation, PHA precursors were synthesized from the reaction of bisaminophenol and three different structures of diacid chlorides as shown in Scheme 5, where the designated reference numbers will apply for this example section.

Scheme 5. Preparation of poly(o-hydroxyamide)s (PHAs) and thermally rearranged polybenzoxazoles (TR-PBDs)

The 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (bisAPAF) (7) as a bisaminophenol was purchased from Central Glass Co. Ltd (Tokyo, Japan). Two rigid aromatic acid chlorides comprized of meta-phenylene (isophthaloyl dichloride, IPCl, 9a) and para-phenylene (terephthaloyl dichloride, TPCl, 9b) moieties from Aldrich Chemical Co. (Milwaukee, Wis., USA) and one relatively flexible acid chloride including hexafluoroisopropane domain ((4,4′-hexafluorois opropylidene bis(benzoyl chloride), 6FCl, 9c) were designed to investigate the structure-property relationship of the precursor PHAs as well as the effect of thermal treatment on the physical properties of TR-PBDs. N-methyl-2-pyrrolidinone (NMP), dimethylformamide (DMF), n-hexane and toluene as solvents, and pyridine, chlorotrimethylsilane (CTMS) and N,N-dimethylaminopyridine (DMAP) as catalysts were obtained from Aldrich Chemical Co. (Milwaukee, Wis., USA) and used without further purification. Thionyl chloride was purchased from Samchun Pure Chemical Co. (Gyeonggi-do, Korea).

As depicted in Scheme 5, three different di(acid chloride) monomers (7) were reacted with equimolar diamine in bis(aminophenol) (9) in the presence of pyridine as a Lewis base to minimize the effect of byproduct of HCl. The exothermic reactions were kept at a reduced temperature to avoid gelation of the polymer solutions, followed by precipitation, filtration and drying. As a catalyst, CTMS had a substantial role to activate the amine group in bis(aminophenol) as well as to protect the hydroxide group, so that high molecular weight polymers could be attained. Consequently, as shown in Table 8 the precursor polymers showed the average molecular weights (Mw) and intrinsic viscosities (ii) exceeding 150,000 g/mole and 0.77 dL/g, respectively.

TABLE 8 Physical and thermal properties of PHAs Polymer Molecular weight Viscosity (dL/g) Structure M_(n) M_(w) M_(w)/M_(n) Inherent Intrinsic 3a 82,400 155,400 1.9 0.73 0.77 3b 36,500 189,400 5.2 0.82 0.79 3c 138,800 262,000 1.9 0.81 0.81

The molecular weight of 3c including bulky and rotational moiety in both repeating units was the highest value of 262,000 g/mole as it was synthesized with small amount of DMAP. These PHAs were characterized by 1H NMR spectra and FT-IR spectra in FIG. 8, which indicated a broad absorption band at 3,400-3,100 cm⁻¹ corresponding to amide and hydroxyl groups, a strong amide carbonyl absorption (C═O) at 1,652 cm⁻¹ and secondary amine absorption (N—H) at 1,540 cm⁻¹. The 1H NMR spectra of the precursor also showed phenolic hydrogen at 10.3 ppm and 10.4 ppm and amide group at 9.7 ppm and 9.8 ppm.

Monomers. High purity (99.9+%) bisAPAF (7) was dried overnight in a vacuum oven at 120° C. before use. Two commercial monocyclic acid chlorides (9a, 9b) were recrystallized under reduced pressure at 120° C. and stored in an inert atmosphere. To synthesize 6FCl (9c), 10 g (30 mmol) of 2,2-bis(4-carboxyphenyl)hexafluoropropane (6FOH) was treated with 25 ml of thionyl chloride in a three-neck round-bottomed flask connected with a dean stark trap and a condenser, followed by heating slowly to 90° C. in an oil bath and stirring for another 6 hours. The 6FCl solidified at reduced pressure and was poured into hexane, filtered and recrystallized. The final 6FCl was obtained as fine white powders after vacuum sublimation at 100° C. This novel monomer structure was confirmed by FT-IR spectra, which indicated a distinct peak at 1,780 cm⁻¹ corresponding to carboxylic acid chloride without any residual peak at 1,704 cm⁻¹ from carboxylic acid.

Precursor polyhydroxylamides (PHAs). The synthesis of three PHA precursors were performed by the same protocol except that different acid chloride monomers were used. In a three-neck round-bottomed flask, 3.66 g (10 mmol) of bisAPAF were dissolved in 20 ml of NMP under a nitrogen atmosphere, followed by dropping both 5.11 ml (40 mmol) of CTMS and 3.24 ml (40 mmol) of pyridine. After the catalysts reacted with the bis(aminophenol) into a silylated form, 10 mmol of each diacid chloride monomer was poured to the solution and stirred vigorously in an ice bath for 4 hours, resulting in a viscous and pale yellowish solution. For the bulky monomer (9c), 4 mmol of DMAP was also added during the exothermic reaction between diamine and diacid chlorides. After terminating the reaction, the solution was precipitated in distilled water, filtered repeatedly and dried at 100° C. under vacuum.

PHA based on (bisAPAF-IPCl) (3a). 1H NMR (300 MHz, DMSO-d₆): 10.3 (OH), 9.7 (NH); FT-IR (powder): (—OH) at 3400-3100 cm⁻¹, amide (C═O) at 1650 cm⁻¹, (N—H) at 1530 cm⁻¹; molecular weight: Mw=155,365, Mn=82,380; Tg (DSC): 211° C.; density 1.45 g/cm³; fractional free volume (FFV) 0.129.

PHA based on (bisAPAF-TPCl) (3b). 1H NMR (300 MHz, DMSO-d₆): 10.3 (OH), 9.7 (NH); FT-IR (powder): (—OH) at 3400-3100 cm⁻¹, amide (C═O) at 1650 cm⁻¹, (N—H) at 1530 cm⁻¹; molecular weight: Mw=189,364, Mn=36.493; Tg (DSC): 261° C.; density 1.46 g/cm³; fractional free volume (FFV) 0.123.

PHA based on (bisAPAF-6FCl) (3c). 1H NMR (300 MHz, DMSO-d₆): 10.3 (OH), 9.8 (NH); FT-IR (powder): (—OH) at 3400-3100 cm⁻¹, amide (C═O) at 1650 cm⁻¹, (N—H) at 1530 cm⁻¹; molecular weight: Mw=262,011, Mn=138,810; Tg (DSC): 258° C.; density 1.40 g/cm³; fractional free volume (FFV) 0.185.

Thermally rearranged polybenzoxazoles (TR-PBOs). PHA powders were dissolved in NMP at 15 wt % concentration, and cast onto well-cleaned glass plates using a doctor blade to control the thickness of precursor films between 30-40 μm. PHA solutions were kept at 80° C. overnight and then heated to 100, 150, 200 and 250° C. in a vacuum oven. The resultant PHA films washed in distilled water were cut to 5×5 cm² size and treated to 350° C. at a heating rate of 5° C./min in a muffled furnace (Lenton, London, UK) so that each precursor (3) can be converted into PBO (5) via intermediate (4) as described in Scheme 1.

PBO based on bisAPAF-IPCl (5a). Tg (DSC): 303° C.; density 1.32 g/cm³; d-spacing: 0.672 nm; fractional free volume (FFV) 0.238.

PBO based on bisAPAF-TPCl (5b). Tg (DSC): 375° C.; density 1.39 g/cm³; d-spacing: 0.614 nm; fractional free volume (FFV) 0.198.

PBO based on bisAPAF-6FCl (5c). Tg (DSC): 326° C.; density 1.27 g/cm³; d-spacing: 0.606 nm; fractional free volume (FFV) 0.282.

Characterizations. Polymer structures of PHAs and TR-PBOs were confirmed by fourier-transformation infrared spectroscopy (FT-IR) (Magna-IR 760 ESP spectroscopy, Thermo Fisher Scientific Inc, Waltham, Mass.) and ¹H-NMR spectra (Varian Mercury Plus 300 MHz spectrometer, Varian, Inc, Palo Alto, Calif.). Elemental analyses (EA) data were obtained by Thermofinnigan EA 1108 at 1000° C. with WO₃/Cu as a catalyst and BBOT (2,5-bis(5-tert-butyl-benzoxazole-2-yl)thiophene) as a standard material. Inherent viscosities of the precursor polymers were measured by using Ubbelohde viscometer (Automatic dilution viscosity measuring system, SI Analytics-Schott instruments, Germany) at 27° C. Molecular weights of precursor PHAs were measured by gel permeation chromatography (Waters GPC system, Milford, Mass.) with PLgel 10 uM Mixed-B LS 300 in 7.5 mm column and Waters 2414 Refractive Index (R1) detector in NMP solution including 0.05 M LiBr at 50° C. on the basis of standard poly(methyl methacrylate) (PMMA).

Conversion of TR-PBO were investigated by thermogravimetric analysis (TGA) with mass spectroscopy (MS), (TGA Q500, TA Instruments, New Castle, Del.) at a rate of 10° C./min under N₂ with Thermo Star GSD 301T (Pfeiffer Vacuum GmbH, Asslar, Germany) to confirm thermal stability of each PHA structure, conversion temperature range with H₂O removal and degradation temperature. Differential scanning calorimetry (DSC, Q20, TA Instruments, New Castle, Del.) was used for measurement of glass transition temperature (Tg) in PHAs and TR-PBOs. Measurement conditions were decided within the degradation temperatures by TGA, at the rate of 5° C./min under N₂ from 180° C. to 300° C. and 400° C. for PHAs and TR-PBOs, respectively. Mechanical properties were characterized to study stress-strain behavior of the polymer samples by using Autograph AGS-J (Shimadzu, Kyoto, Japan) with two film specimens of each sample (1¼″ High ASTM D-638 Type). Wide angle X-ray Diffraction (WAXD) was applied to investigate d-spacing of PHAs and TR-PBOs by X-ray diffractometer (Rigaku Denki D/MAX-2500, Rigaku, Japan). The threshold values in the 20 range of 5°-80° with a scan rate of 5° C./min by 1.54 Å wavelength of Cu Kα radiation source was employed to get the average intermolecular distance through the following Bragg's equation:

$\begin{matrix} {d = \frac{n\; \lambda}{2\; \sin \; \theta}} & (1) \end{matrix}$

where λ is the X-ray wavelength and θ is the angle of maximum intensity in the amorphous halo exhibited by the polymer. Density was obtained to buoyancy method using Sartorious LA 120S analytical balance in water at 27° C. Van der Walls volumes corresponding to each polymer structures were calculated by Bondi's group contribution theory to obtain the fractional free volume (FFV, Vf), which was calculated using the following equation:

V _(f)=(V−1.3V _(w))/V  (2)

where V is the polymer-specific volume and V_(w) is the specific Van der Waals volume.

PALS Experimental. PALS was used to investigate the pore size and relative pore concentration within the PHAs and TR-PBOs. The size of the free volume elements within the polymers can then be related to the transport properties of the membranes. PALS measurements were undertaken on an EG&G Ortec fast-fast coincidence system using a vacuum cell equipped with a heat controller. Long lifetimes were collected by setting the range of the time-to-amplitude converter to 200 ns and removing the coincident unit to increase count rates. Each file consisted of 106 integrated counts and a minimum of 5 files were collected for each sample or at each temperature. The FWHM resolution of the instrument was determined to be 240 ps when measured with ⁶⁰Co.

The positron source was prepared with 50 μCi of ²²Na which was dried onto 2.54 μm thick Ti foil and required no background subtraction. The TR membranes were stacked to 2 mm thickness and placed on each side of the positron source. The sample and source were then placed in the vacuum cell and brought to 5×10⁻⁴ Pa. The samples were all initially measured at room temperature (20° C.) under vacuum. PALS was also measured on sample 5c from 30 to 230° C. at 20 or 30° C. intervals and then returned to 30° C. to ensure there were no permanent changes in the free volume due to the heating regime.

The PALS data was deconvoluted using a four component fit with LTv9 software by fixing the first lifetime (τ₁) to 0.125 ns due to annihilation of the para positronium (p-Ps) and freeing the second lifetime (τ₂) to ˜0.4 ns due to free annihilation. Therefore two ortho-positronium (o-Ps) components (τ₃ and τ₄) were associated with the bimodal porosity of the PHAs and TR-PBOs. The t₃ shorter lifetimes were converted to pore sizes using the Tao-Eldrup semi-empirical formula. The longer lifetimes and the lifetimes at high temperatures were calculated using the rectangular Tao-Eldrup (RTE) model.

Gas permeation properties. Gas permeation properties of PHAs and TR-PBOs were measured by both constant-volume method, so called as ‘time-lag method’, at room temperature and constant-pressure method at high temperature. Gas permeabilities and diffusivities were determined at the steady-state pressure increments in-between vacuum and 760 Torr over the membranes for six representative permanent gases (e.g. He, H₂, CO₂, O₂, N₂, and CH₄) at 300K as follows:

$\begin{matrix} {P = {\frac{p}{t}\left( \frac{{VT}_{0}l}{P_{0}{TA}\; \Delta \; P} \right)}} & (3) \end{matrix}$

where P is the permeability represented in Barrer, dp/dt is the rate of pressure rise under the steady state, V (cm³) is the downstream volume, L (cm) is the membrane thickness, T (K) is the measurement temperature, Δp (cmHg) is the pressure difference, A (cm²) is the effective membrane area, and P₀ and T₀ are the standard pressure and temperature, respectively.

Temperature dependence of the polymers were investigated for H₂ and CO₂ targeted to a specific application. Based on a careful consideration on safety issues in a convection oven, the membrane cell was connected to pre-heated gas chamber equipped with several valves and manometers (Baratron 722 and 626A, MKS instrument, MA, USA). Gas permeances were recorded at each pressurized gas up to 20 bar at the temperature range of 300-493 K and converted to gas permeabilities as follows:

$\begin{matrix} {P = \frac{Q \times l}{A \times \Delta \; P}} & (4) \end{matrix}$

where Q (cm³/min) is the gas flow rate at downstream, l (cm) is the membrane thickness, Δp is the pressure difference (cmHg), and A (cm²) is the effective membrane area. Permselectivity (α_(1/2)) is defined as the ratio of the two gas permeabilities.

Thermal rearrangements. Thermal conversion of PHAs into TR-PBOs occur by the attack of the hydroxide group into the adjacent amide carbonyl group as thermal treatment accelerates polymer chain mobility and reduces the activation energy for intra- and inter-molecular reactions even at solid state. The intermediate structures composed of fused five-membered rings undergo dehydration to be conjugated with aromatic rings at around 350° C. The intramolecular reactions were investigated conveniently as tracing the weight change of PHAs and identifying the evolved gases, which could be performed by mass spectroscopy connected to thermogravimetric analysis.

As shown in FIGS. 9( a)-9(c), PHA precursor 3c has weight reduction of around 6 wt % in the range of 240 to 380° C. Thermal conversion and decomposition of 3a and 3b possessing the same chemical composition but different main chain formation of the phenyl rings displayed the same trend, however, 3c which includes hexafluoroisopropylidene group, C(CF₃)₂, has the highest initial conversion temperature (T_(ci)). Note that the weight drops coincide with the intensity variation of H₂O at the same temperature range without detection on other mass numbers in mass spectroscopy. As a result, the thermally treated polymers were confirmed by the newly detected FT-IR peaks (FIG. 10) at 1,054 cm⁻¹ (C-0 stretching) and 1,475 cm⁻¹ (C═N stretching) in benzoxazole ring, as well as the peak disappearances at 3,400-3,100 and 1,650 cm⁻¹ corresponding to hydroxyl groups and amide carbonyl, respectively. The elemental analyses of PHAs and TR-PBOs in Table 9 generally agreed well with the calculated values for the proposed structures.

TABLE 9 Molecular Polymer formula of H N Total structure repeating unit C (wt %) (wt %) (wt %) (wt %) PHA 3a C₂₃H₁₄N₂O₄F₆ 59.83 2.60 6.49 68.92 (55.65)* (2.85)* (5.65)* (64.15)* 3b C₂₃H₁₄N₂O₄F₆ 58.29 3.08 6.54 67.91 (55.65)* (2.85)* (5.65)* (64.15)* 3c C₃₂H₁₈N₂O₄F₁₂ 55.79 2.83 4.78 63.4 (53.19)* (2.52)* (3.88)* (59.59)* *calculated values

Without intending to be bound by a particular theory, hydroxyl groups in PHA are more flexible around the aromatic amide linkage including secondary amine than those in HPI composed of tertiary amine, thus PHAs can be thermally rearranged at temperatures 100° C. lower than HPIs. Moreover, differences in the main chain and the following conversion routes can bring out physical properties in the polymer matrix.

Physical properties of TR-polymers. The most significant effect in the thermal rearrangement of these polymers is the change of the physical properties such as intermolecular distance, internal surface area, and fractional free volume (FFV) while the changes in chemical structures contribute to the variation of glass transition temperatures (Tg) and thermal degradation temperatures (Td). In the case of TR-α-PBOs, their free volume increments were almost doubled so that the FFV could reach to 30% or more. Here, TR-β-PBOs (5a-5c) obtained by thermal treatment at 350° C. exhibited increased free volume compared to PHAs, but were different from those of TR-α-PBOs. Referring to FIGS. 12( a) and 12(b), the d-spacing values calculated by the average two theta values from the WAXD patterns were enlarged with increasing thermal treatment temperature. Apart from polymers including bulky and rotational polar C(CF₃)₂ groups which have several strong crystalline peaks, PHA and PBO linked to para-phenylene (3b to 5b) have larger intermolecular distances compared to meta-phenylene polymers (3a to 5a).

Polymer densities and the resulting free volume calculations showed a good correlation with the trend in d-spacing values. As described in Table 10, the densities of PHAs were 1.4 gcm⁻¹ (3c) to 1.46 gcm⁻¹ (3b) according to their main chain components and stiffness like those of conventional aromatic polymers.

TABLE 10 Tem- d- V_(w) perature 2theta spacing Density V (cm³/ FFV Polymers (° C.) (degree) (nm) (g/cm³) (cm³/g) g) (—) 3a 250 13.88 0.630 1.45 0.690 0.462 0.129 4a 300 13.65 0.648 1.45 0.690 0.445 0.161 5a 350 13.2 0.672 1.32 0.758 0.444 0.238 3b 250 14.25 0.590 1.46 0.685 0.462 0.123 4b 300 14.55 0.610 1.43 0.699 0.445 0.172 5b 350 14.40 0.614 1.39 0.719 0.444 0.198 3c 250 15.50 0.583 1.40 0.714 0.448 0.185 4c 300 14.99 0.606 1.33 0.752 0.436 0.246 5c 350 15.35 0.606 1.27 0.787 0.435 0.282

As thermal treatment provides enough activation energy for chain distortion and conversion, polymer densities diminished 5-10% in TRβ-PBOs. The para-linked TR-β-PBO (5b) retained the highest density of 1.39 gcm⁻¹ owing to the pristine packed morphology at the same thermal treatment protocol whereas the bulky sample (5c) showed a significant density drop (9.3%) as well as density itself (1.27 gcm⁻¹), because these polymers have small thermal shrinkages less than 4%. Fractional free volume (FFV) changes are also notable in that the improved values are not anticipated for thermostable aromatic polymers. In the FFV calculation, the samples (4) (Scheme 5) treated at 300° C. were considered discretionarily to exist in the state of intermediate structures, where van der Walls volumes were close to those of TR-β-PBOs, although it was difficult to detect in the reaction. FFVs increased generally in order of 250(3)<300(4)<350(5) and para-phenylene (b)<meta-phenylene (a)<hexafluoroisopropylidene (c) for treatment temperatures and structures, respectively. Although the FFVs were smaller than those of TR-α-PBOs (0.28-0.35), they are comparative to FFVs of amorphous fluoropolymers such as Hyflon AD series and Cytop™ as well as polymers with intrinsic microporosity (PIMs), which are promising polymers for separation and storages with FFVs of 0.19-0.33. High free volumes in those polymer matrixes contribute to high sorption capability in their internal surfaces as well as provide diffusion pathways for fast transport through the polymers.

Positron annihilation lifetime spectroscopy (PALS) confirmed these important characteristics by visualizing the free volume elements as pore size distributions and intensities. As a positron can live in the porous space with low electron density, the longer lifetime of the positron, the larger the pore size in the material. Computational software such as LTv9 and PAScual can exhibit several individual lifetimes with their fit variances. As shown in FIGS. 11( a), 11(b), and 11(c) and Table 11, PHAs and TR-β-PBOs have bimodal distributions as other high free volume polymers (e.g. PTMSP, PIM, TR-α-PBOs) have shown.

TABLE 11 Treatment Cavity Cavity Temperature Diameter Intensity, I₃ Diameter Intensity, I₄ Polymers (° C.) τ₃ (ns) (nm) (%) τ₄ (ns) (nm) (%) 3a 250 0.82 ± 0.13 0.27 ± 0.06 11.09 ± 4.12  2.51 ± 0.03 0.655 ± 0.004 18.57 ± 0.45 4a 300 0.90 ± 0.10 0.30 ± 0.04 9.93 ± 1.99 2.92 ± 0.03 0.716 ± 0.004 21.97 ± 0.37 5a 350 0.99 ± 0.09 0.33 ± 0.03 8.44 ± 1.05 3.03 ± 0.03 0.731 ± 0.004 19.14 ± 0.48 3b 250 1.01 ± 0.11 0.34 ± 0.04 8.07 ± 0.77 2.92 ± 0.04 0.716 ± 0.006 16.09 ± 0.51 4b 300 0.96 ± 0.11 0.32 ± 0.04 8.68 ± 0.90 3.13 ± 0.03 0.744 ± 0.004 19.51 ± 0.44 5b 350 0.96 ± 0.06 0.32 ± 0.02 9.68 ± 0.72 3.31 ± 0.03 0.767 ± 0.004 21.00 ± 0.33 3c 250 0.85 ± 0.08 0.28 ± 0.03 9.10 ± 1.34 2.80 ± 0.02 0.699 ± 0.003 17.49 ± 0.22 4c 300 1.22 ± 0.21 0.40 ± 0.06 7.09 ± 0.49 3.60 ± 0.05 0.802 ± 0.006 22.16 ± 1.00 5c 350 1.24 ± 0.10 0.40 ± 0.03 8.88 ± 0.54 3.79 ± 0.03 0.825 ± 0.004 21.54 ± 0.76

Lifetime of PHAs, which have 0.82-1.01 ns and 2.50-2.92 ns for τ₃ and τ₄, respectively, ascended to 0.96-1.24 ns and 3.03-3.79 ns in TR-β-PBOs, corresponding to the cavity size of 0.32-0.4 nm and 0.73-0.83 nm, generally in the order of 3a<3c<3b<5a<5b<5c. The significant changes in both smaller and larger cavities indicates that the chain rearrangement enlarged the internal free volumes within the polymer matrix. Therefore, the increased free volume elements evolved by thermal rearrangement of HPAs are expected to have superior transport properties to small gas and vapor molecules. Despite the two cavities sizes being comparable to those of TR-α-PBOs (0.38 and 0.9 nm), they were slightly smaller, therefore, it would be predicted that their diffusion selectivity for small gas penetrants would be improved although diffusion through the polymers could be restricted to the high free volume polymers.

Gas separation properties. As transport of gas molecules through polymers were governed by well-known ‘solution-diffusion’ mechanism, TR-PBOs retaining high free volume elements, as elucidated by X-ray diffraction, density, FFV and PALS, exhibited superior permeabilities for small gas molecules compared to conventional glassy polymers. As can be seen in Table 12, gas permeabilities of PHAs (3a-3c) showed relatively low gas permeabilities and reasonably high ideal selectivities.

TABLE 12 Gas permeation properties of PHAs and TR-β-PBOs at 300 K Polymer Gas permeability Ideal selectivity Structures He H₂ CO₂ O₂ N₂ CH₄ O₂/N₂ CO₂/N₂ CO₂/CH₄ H₂/CO₂ H₂/CH₄ N₂/CH₂ 3a 9.2 5.4 2.1 0.47 0.09 0.03 5.2 23 69 2.6 180 3.0 3b 20 14 4.7 1.4 0.2 0.1 6.3 21 43 3.0 128 2.1 3c 106 75 32 8.0 1.6 0.83 5.1 20 40 2.3 93 2.0 5a 70 60 23 5.7 1.0 0.5 5.6 23 45 2.6 115 2.0 5b 82 85 53 11 2.3 1.4 4.9 23 39 1.6 59 1.6 5c 251 255 199 45 11 6.4 4.2 18 31 1.3 40 1.7 Pressure: 760 Torr. Temperature: 300 K

H₂ permeability of 3b membrane exhibited 14 Barrer while ideal selectivity of hydrogen over carbon dioxide is about 3. On the other hand, 3b was treated at 350° C. to become TR-β-PBO membrane (5b), H₂ permeability was improved 6 times while selectivity was reduced in half. This result is consistent with an increase in free volume as the larger gases (kinetic diameter>3.2 Å), which are originally subject to larger energy barriers due to size exclusion within 3b, achieve enhanced diffusivities within 5b as almost barrier-free pathways are presented. Smaller gases (kinetic diameter<3.2 Å) on the other hand, achieve enhanced solubilities at a detriment to diffusivity where an increase in free volume creates ideal pore-filling space. Therefore, permselectivities for small gas pairs were partially mitigated. Notice that H₂ permeability of 5a was about 60 Barrer and selectivity of H₂/CO₂ was still about 2.6. In 5a and 5b, as evidenced in Table 10 and FIGS. 11( a)-(c), the difference in their size exclusion capabilities resulted in higher permselectivities in the former although they showed similar He permeability. 5c membranes showed relatively high permeabilities for six representative gases among the TR membranes tested because of the high free volume and large cavity sizes. For this membrane, CO₂ permeability was about 200 Barrer and the selectivity over nitrogen and methane was about 18 and 31, respectively. As shown in FIGS. 13( a)-(c), there was a trade-off relationship between gas permeability and permselectivity, the separation performances approached but did not surpass the Robeson's upper bounds. See [L. M. Robeson, Correlation of separation factor versus permeability for polymeric membranes, J. Membr. Sci., 1991, 62, 165; L. M. Robeson, The upper bound revisited, J. Membr. Sci., 2008, 320, 390].

Temperature dependences on physical properties and separation performances. Though aromatic polymers usually possess glass transition temperatures greater than 150° C., the ability to utilize them at elevated temperatures has typically been restricted because of the long-term stability and performance reduction. While common glassy polymers have typically shown two different rates of increasing free volume at the boundary of glass transition temperature (Tg), high free volume polymers represent very ambiguous behaviors in their patterns. Even though these polymers have very high Tg, their cavity sizes in bimodal distributions as well as unimodal distributions have a threshold at a certain temperature under Tg and start to decline: for instance, AF 2400 and PIMs showed thresholds at 170° C. and 90-110° C., respectively. The same phenomena were detected in TR-PBOs. FIGS. 14( a) and (b) represent the changes of cavity diameters of 5c membranes in terms of τ₃ and τ₄ as a function of temperature ranging from 30 to 220° C. measured using PALS. Noticeably, T₃, representing the small cavity, as well as τ₄, showing the large cavity, increase as a function of temperature. However, τ₄ decreases at temperatures above 200° C. for the 5c membrane.

FIG. 15 (a) shows the permeability and selectivity variation of H₂ and CO₂ for TR-β-PBO membranes (5a-5c) as a function of temperature from 30 to 230° C. For the 5c membrane, gas permeability of hydrogen and carbon dioxide was tested at Los Alamos National Laboratory to confirm the accuracy of the gas permeability measured in our laboratory. The results are very consistent when using the same membranes measured at temperatures up to 230° C. In general, permeabilities of H₂ and CO₂ both increase with temperature for all three membranes. An increase in permeability of gases is understandable from the view point of the changes in cavity size as a function of temperature as was discussed in connection with FIG. 14. Note that the increase of H₂ permeability is pronounced in comparison to that of CO₂ resulting in the increase of selectivity of H₂/CO₂ because the reduction of CO₂ solubility at higher temperatures are more critical than those of H₂ and thus counterbalance the higher diffusion increase of CO₂. Activation energies of H₂ and CO₂ derived from the Arrhenius plot data such as FIG. 15( a) are 10.6, 6.3 kJ/mol·K in 5a, and 7.8, 2.5 kJ/mol·K in 5c, respectively. In general the activation energies are in the order of H₂ (5a)>H₂ (5b)>H₂ (5c)>CO₂ (5a)>CO₂ (5c)>CO₂ (5b) because gas molecules passing through smaller cavities are easily disturbed for permeation and thus the temperature dependence on permeation are more significant. As a result, 5a showed the highest separation performance with 205.8 Barrer of H₂ permeability and 6.2 of H₂/CO₂ selectivity at 215° C.

Therefore, H₂/CO₂ separation performances are surprisingly improved in both permeability and selectivity at elevated temperatures which is unique to other gas separations. This peculiar temperature dependence of gas permeation has already been noticed by Dye et al. Thermal energy prompts the increases in diffusivity and thus the permeability of most gases for TR-β-PBO membranes, contributing to the exceeding upper bound as can be seen in FIG. 15( b). Here, TR-β-PBOs showed better separation performances than TR-PBI reported in Han et al., J. Membrane Sci., Vol. 357, 2010, pp. 143-151. Although there needs to be further optimization and improvement in terms of selectivity for TR-β-PBO membranes, this result shows the potential for TR-β-PBO membranes for the syn gas separation application. The data from FIGS. 15( a) and 15(b) is listed in the following Tables 13 to 16.

TABLE 13 TR-PBI T (° C.) 1000/K He H2 CO2 O2 N2 CH4 CO α (H2/CO2) Permeability 30.00 3.30 868.44 1778.83 1624.09 336.64 61.93 35.26 93.60 1.10 60.00 3.00 905.92 1757.48 1598.92 327.05 76.79 54.08 119.66 1.1 90.00 2.75 907.26 1615.55 1267.72 289.49 82.04 70.58 129.44 1.27 120.00 2.54 893.61 1551.69 1004.15 262.58 85.28 81.43 133.40 1.55 Diffusivity 30.00 3.30 1.13E−06 1.12E−06 2.98E−07 7.48E−07  8.2E−07 4.11E−08 4.82E−07 3.76 60.00 3.00 1.01E−06 8.62E−07 4.47E−07 1.06E−06 9.78E−07 1.02E−07 5.12E−07 1.92 90.00 2.75 9.99E−07 1.05E−06 5.21E−07 8.22E−07 1.52E−06 2.13E−07 1.67E−06 2.02 120.00 2.54  1.1E−06 1.01E−06 6.33E−07 7.17E−07 2.19E−06 3.22E−07 2.23E−06 1.59 Solubility 30.00 3.30 0.0772 0.1586 0.5453 0.0450 0.0076 0.0858 0.0194 0.29 60.00 3.00 0.0896 0.2039 0.3573 0.0310 0.0079 0.0529 0.0234 0.57 90.00 2.75 0.0908 0.1535 0.2433 0.0352 0.0054 0.0332 0.0078 0.63 120.00 2.54 0.0814 0.1536 0.1585 0.0366 0.0039 0.0253 0.0060 0.97

TABLE 14 TR-b-PBO 6F-IP (5a) ° C. 1000/K H2 CO2 α (H2/CO2) Permeability 30 3.30 44 6 8 60 3.00 61 17 3.67 90 2.75 83 22 3.75 120 2.54 111 22 5 150 2.36 139 28 5 180 2.21 178 28 6.4 200 2.11 195 33 5.83 210 2.07 206 33 6.17 225 2.01 222 39 5.71

TABLE 15 6F-TP (5b) ° C. 1000/K H2 CO2 α (H2/CO2) Permeability 30 3.30 191 60 3.16 60 3.00 130 60 2.16 90 2.75 143 60 2.37 120 2.54 168 64 2.65 150 2.36 200 67 3 180 2.21 238 70 3.41 200 2.11 270 76 3.54 210 2.07 280 76 3.67 225 2.01 305 79 3.84

TABLE 16 6F-6F (5c) ° C. 1000/K H2 CO2 α (H2/CO2) Permeability 30 3.30 144 95 1.5 60 3.00 210 124 1.69 90 2.75 248 130 1.9 120 2.54 321 130 2.46 150 2.36 372 133 2.79 180 2.21 426 140 3.05 200 2.11 451 146 3.09 210 2.07 464 156 2.98 225 2.01 499 149 3.34

As apparent from the foregoing, the benzoxazole-based polymers according to the present invention are suitable for use in various separation membranes, in particular, separation membranes applicable to small gases. The stability of these membrane materials at high temperatures makes them suitable for various gas separation applications at temperatures higher than most current commercially available polymeric membranes. In many gas separations applications using conventional polymeric membrane materials, the feed gas to the membrane must be cooled, because of temperature/stability limitations of most current polymer membrane materials. Such cooling is not required as a result of the present invention.

For example, for separations of air (involving principally separation of O₂ from N₂), the elevated pressure gas from a compressor output is very hot, possibly at temperatures as high as 100 to 200C, depending on the type of compressor. In air separations used to enrich N₂ by onboard inert gas generation systems (commonly referred to as OBIGGS), the feed air to the membrane is obtained from a compression stage of a turbine (in an aircraft engine), where temperature of the gas can be as high as about 200 C. Using conventional polymeric membrane materials, which cannot tolerate such high temperatures, a heat exchanger must be used to first cool the feed gas to the membrane. Need for such after-cooling could be largely avoided for the membrane polymers of the present invention, since these polymers show much better high temperature stability. For example, as shown by the TGA curves in FIGS. 1 and 9, decomposition of the polymer does not begin to occur until temperatures exceed approximately 400 to 500° C. Hot operating environments may also be encountered, where the polymers of the present invention may be useful by virtue of their stability at high temperature, such as under the hood in the engine compartments of diesel engines. For diesel engines, it may be desirable to use a membrane to remove some of the O₂ from feed air, so as to reduce the nitrogen oxides (NOx), which are harmful pollutants normally contained in the exhaust gases from diesel engines operating on normal air.

Further, many H₂ separations are employed in a variety of chemical processing and refinery process applications, such as separations of H₂ from CH₄ and other light hydrocarbons, H₂ from CO, H₂ from N₂, and the like. Many of these process gas streams operate at very high temperatures, such that use of convention polymer membrane materials requires that the feed gas to the membranes must be cooled prior to the gas contacting the membrane. In refining processes, for example to name just a few of many, hydro-treating processes may operate at temperatures ranging from about 200° C. to over 400° C., isomerization processes may operate in the range of about 150° C. to about 200° C., catalytic reforming processes may operate at even higher temperatures, for example, in the vicinity of 400-500° C. These are just a few of many examples of potential uses of the polymers of the present invention, which are made possible by their high temperature stability. 

1. A method of separating H₂ and CO₂ from a gas mixture, the method comprising: passing the gas mixture comprising H₂ and CO₂ through a benzoxazole-based polymer membrane at a temperature of from about 30° C. to about 400° C., wherein the benzoxazole-based polymer membrane is represented by the formula:

wherein Ar is a bivalent C₅-C₂₄ arylene group or a bivalent C₅-C₂₄ heterocyclic ring, which is substituted or unsubstituted with at least one substituent selected from the group consisting of C₁-C₁₀ alkyl, C₁-C₁₀ alkoxy, C₁-C₁₀ haloalkyl and C₁-C₁₀ haloalkoxy, or two or more of which are fused together to form a condensation ring, or covalently bonded to each other via a functional group selected from the group consisting of O, S, C(═O), CH(OH), S(═O)₂, Si(CH₃)₂, (CH₂)_(p) (in which 1≦p≦10), (CF₂)_(q) (in which 1≦q≦10), C(CH₃)₂, C(CF₃)₂ and C(═O)NH; Q is a single bond, O, S, C(═O), CH(OH), S(═O)₂, Si(CH₃)₂, (CH₂)_(p) (in which 1≦p≦10), (CF₂)_(q) (in which 1≦q≦10), C(CH₃)₂, C(CF₃)₂, C(═O)NH, C(CH₃)(CF₃), C₁-C₆ alkyl-substituted phenyl or C₁-C₆ haloalkyl-substituted phenyl in which Q is linked to opposite both phenyl rings in the position of m-m, m-p, p-m or p-p; and n is an integer of 20 to
 400. 2. The method of claim 1, wherein Ar is selected from the following compounds:

wherein X is O, S, C(═O), CH(OH), S(═O)₂, Si(CH₃)₂, (CH₂)_(p) (in which 1≦p≦10), (CF₂)_(q) (in which 1≦q≦10), C(CH₃)₂, C(CF₃)₂, or C(═O)NH; Y is O, S or C(═O); and Z₁, Z₂ and Z₃ are identical or different and are O, N or S.
 3. The method of claim 1, wherein Ar is selected from the following compounds:


4. The method of claim 1, wherein Q is a single bond, C(CH₃)₂, C(CF₃)₂, C(═O)NH, C(CH₃)(CF₃),


5. The method of claim 1, wherein Ar is

and Q is C(CF₃)₂.
 6. A method of separating a pair of gasses from a gas mixture comprising the pair of gases, the method comprising: passing the gas mixture through a benzoxazole-based polymer membrane at a temperature of from about 30° C. to about 400° C., wherein the benzoxazole-based polymer membrane is represented by the formula:

wherein Ar is a bivalent C₅-C₂₄ arylene group or a bivalent C₅-C₂₄ heterocyclic ring, which is substituted or unsubstituted with at least one substituent selected from the group consisting of C₁-C₁₀ alkyl, C₁-C₁₀ alkoxy, C₁-C₁₀ haloalkyl and C₁-C₁₀ haloalkoxy, or two or more of which are fused together to form a condensation ring, or covalently bonded to each other via a functional group selected from the group consisting of O, S, C(═O), CH(OH), S(═O)₂, Si(CH₃)₂, (CH₂)_(p) (in which 1≦p≦10), (CF₂)_(q) (in which 1≦q≦10), C(CH₃)₂, C(CF₃)₂ and C(═O)NH; Q is a single bond, O, S, C(═O), CH(OH), S(═O)₂, Si(CH₃)₂, (CH₂)_(p) (in which 1≦p≦10), (CF₂)_(q) (in which 1≦q≦10), C(CH₃)₂, C(CF₃)₂, C(═O)NH, C(CH₃)(CF₃), C₁-C₆ alkyl-substituted phenyl or C₁-C₆ haloalkyl-substituted phenyl in which Q is linked to opposite both phenyl rings in the position of m-m, m-p, p-m or p-p; and n is an integer of 20 to 400, wherein the gas pair is selected from the group consisting of H₂/CH₄, H₂/CO₂, H₂/N₂, He/N₂, O₂/N₂, CO₂/N₂, and CO₂/CH₄.
 7. The method of claim 6, wherein Ar is selected from the following compounds:

wherein X is O, S, C(═O), CH(OH), S(═O)₂, Si(CH₃)₂, (CH₂)_(p) (in which 1≦p≦10), (CF₂)_(q) (in which 1≦q≦10), C(CH₃)₂, C(CF₃)₂, or C(═O)NH; Y is O, S or C(═O); and Z₁, Z₂ and Z₃ are identical or different and are O, N or S.
 8. The method of claim 6, wherein Ar is selected from the following compounds:


9. The method of claim 6, wherein Q is a single bond, C(CH₃)₂, C(CF₃)₂, C(═O)NH, C(CH₃)(CF₃),


10. The method of claim 6, wherein Ar is

and Q is C(CF₃)₂.
 11. A gas separation membrane comprising polybenzoxazole (TR-β-PBO) represented by the formula:

wherein Ar is a bivalent C₅-C₂₄ arylene group or a bivalent C₅-C₂₄ heterocyclic ring, which is substituted or unsubstituted with at least one substituent selected from the group consisting of C₁-C₁₀ alkyl, C₁-C₁₀ alkoxy, C₁-C₁₀ haloalkyl and C₁-C₁₀ haloalkoxy, or two or more of which are fused together to form a condensation ring, or covalently bonded to each other via a functional group selected from the group consisting of O, S, C(═O), CH(OH), S(═O)₂, Si(CH₃)₂, (CH₂)_(p) (in which 1≦p≦10), (CF₂)_(q) (in which 1≦q≦10), C(CH₃)₂, C(CF₃)₂ and C(═O)NH; Q is a single bond, O, S, C(═O), CH(OH), S(═O)₂, Si(CH₃)₂, (CH₂)_(p) (in which 1≦p≦10), (CF₂)_(q) (in which 1≦q≦10), C(CH₃)₂, C(CF₃)₂, C(═O)NH, C(CH₃)(CF₃), C₁-C₆ alkyl-substituted phenyl or C₁-C₆ haloalkyl-substituted phenyl in which Q is linked to opposite both phenyl rings in the position of m-m, m-p, p-m or p-p; and n is an integer of 20 to 400, wherein the polybenzoxazole has a weight average molecular weight of from more than about 50,000 Da to about 300,000 Da.
 12. The gas separation membrane of claim 11, wherein the polybenzoxazole has a weight average molecular weight of from more than about 50,000 Da to about 200,000 Da.
 13. The method of claim 1 wherein the gas mixture is passed through the benzoxazole-based polymer membrane at a temperature of from about 200° C. to about 350° C.
 14. The method of claim 6 wherein the gas mixture is passed through the benzoxazole-based polymer membrane at a temperature of from about 200° C. to about 350° C.
 15. The method of claim 1, wherein Ar is

and Q is C(CF₃)₂.
 16. The method of claim 1, wherein Ar is

and Q is C(CF₃)₂.
 17. The method of claim 6, wherein Ar is

and Q is C(CF₃)₂.
 18. The method of claim 6, wherein Ar is

and Q is C(CF₃)₂.
 19. The gas separation membrane of claim 11, wherein Ar is

and Q is C(CF₃)₂.
 20. The gas separation membrane of claim 11, wherein Ar is

and Q is C(CF₃)₂.
 21. The gas separation membrane of claim 11, wherein Ar is

and Q is C(CF₃)₂. 